If POPULAR SCIENCE

Star (Christchurch), Issue 19258, 20 December 1930, Page 19 (Supplement)

If POPULAR SCIENCE

(By

J. J. S. CORNES,

8.A., B.Sc.)

RADIUM AND X-RAYS

W%en the element radium was discovered, nearly thirty years ago, it excited widespread interest and brought fame to its discoverer, Madame Curie. Radium bromide is a white salt, and the few ounces which have been prepared are worth a king’s ransom. It is, therefore, fortunate that a very small quantity will serve anyone’s purpose, and that, having paid so much for it, no one wastes it. As a matter of fact, it slowly wastes itself, and in that peculiarity, indeed, lies its value. The insignificant quantity which has been produced is gradually disappearing, but it has already enabled discoveries to be made which have changed the aspect of physical science. There are certain periods in the history of science when a discovery or group of discoveries alters the whole trend of thought. Startling as the new facts and phenomena may be, they are overshadowed by the far-reaching ideas they suggest and by the influence they exert in modifying views that have become set and almost sacred. Old mental pictures, fruitful in their day, in indicating the direction of further experiment and reasoning, are wiped out, and for a time scientific men are busy painting, with tentative and hesitating strokes, the new picture of the physical universe. Of such a nature are the discoveries which have risen out of the study of radium. For this element, occurring very widely, though in minute quantities throughout the earth’s crust, is of relatively small importance considered alone. But attempts to explain its properties have shed a new light on the elusive phenomena of electricity, and have shaken the foundations on which our most elementary notions of chemistry were laid. Like most profound problems, the real solution was attained by'no single series of experiment, and by no single experimenter. To explain radio-activity we must first pursue several lines of enquiry, and then correlate them. The Discharge of Electricity Through Gases. Air and other gases are, at ordinary temperatures and pressures, and when dry, non-conductors of electricity. This does not mean that electricity cannot be induced to pass through them at all, but that an enormous electromotive force is required for that purpose. On this fact is based the possi-

m u ® ® ® ® ® ® ® ® ® s a ® e w ® ® ® s bility of transmitting electrical power over long distances by means of bare wires, for, though some leakage does take place, it is mostly through the insulated supports or the thin film of moisture which covers them in wet weather. In order that a spark may pass between two balls one inch apart, in air, an electromotive force of something like 100,000 volts is required—though if points instead of balls are used discharge takes place more readily. A highly rarefied gas conducts more easily. If it is contained in a tube which can be gradually exhausted, the electrodes by which the alternating current from an induction coil enters may be placed several inches apart. At first there is no discharge, but as exhaustion proceeds a broad band of light appears between the electrodes, which, as the pump is worked, widens until it fills the tube. The colour depends upon the nature of the gas. (We are all becoming increasingly familiar with the crimson advertising signs about the city, brilliant even in daylight—these are tubes of rarefied neon gas glowing by electrical discharge.) At one stage there is a flickering appearance owing to the concentration of the light in thin layers which fill the tube from end to end. If, in this condition, the tube is sealed off, it forms one of the wellknown vacuum tubes which give such beautiful effects when connected up to induction coil or influence machine. As the vacuum becomes higher a dark space forms around one of the electrodes —called the cat.hode-~and this space increases as the quantity of gas IS USE Hl® Si*]®®®® S 3 IS®®®®®® El

in the tube becomes less, until it fills the tube, the walls of which now glow with a faint greenish light. Finally, when exhaustion is pushed to the fullest extent the electricity refuses to pass, showing that the gaseous matter originally in the tube was the bearer of electricity. (The broad band of light forms when the pressure falls from 760 to 10 millimetres of mercury, the striae or flickering layers at 3 m.m., while tom dark space fills the tube at 0.03 when only one twenty-five-thousandth of the air is left.) From the middle of last century these effects excited considerable interest, and many beautiful experiments were devised. , Sir Wiliam Crookes placed a small wheel with vanes mounted on an axle in the middle of the tube. The fact that this wheel rotated when the dark space reached it showed that actual particles of matter were projected across the space between the cathode and walls of the tube, bom* s® ® ®a®®®®® ®®® e® s m

® m m ® ® ® ® m ® m ® ® ® ® m ® barding the wheel. The fixed wheel was replaced by one having its axle delicately mounted on two glass rails running the length of the tube. The wheel rolled from end to end, and when the current was reversed ran back. Crookes showed that if a magnet was held near the tube, the stream of particles would be bent out of its original direction, so that the wheel no longer turned. If the stream wqs concentrated on a small piece of platinum by means of a concave cathode, the platinum was raised to red heat by bombardment. The deflection by the magnet is, just what would occur if the stream were composed of tiny particles bearing charges of negative elec-tricity-—equivalent to an electricL. current. In 1894 Lenard showed that the rays penetrated a thin aluminium partition in the tube, but were absorbed by heavier metals, such as lead. This explains why aluminium is used for anode and cathode, and why a heavy metal such as platinum becomes incandescent on bombardment. Further investigation, mainly by Sir J. J. Thomson, and his pupils, proved that these cathode rays produced the same effect whatever the gas in the tube. Thomson, and later Millikan, haik measured the mass of the flying particles as about one-eighteen thousandth that of the hydrogen atom, and their velocity as reaching 60,000 miles per second. In 1897 Sir J. J. Thomson advanced the view that these “ electrons ” are actually present in the atoms of the gas, or of any other kind of matter —that atoms, indeed, are groups of these electrons—and that they are torn off the gaseous Atom* during the passage of the current. We must now consider another kind of radiation produced when the flying electrons strike an object in thca; path X-Rays (Rontgen Rays). In 1896 Professor Rontgen was using one of the tubes just described when he found that some photographic plates in a drawer of the bench became fogged. From this he was led to the discovery of the X-rays produced from any solid body when bom* 1 barded by a stream of “ radiant matter,” or “electrons.” Usually a disc of heavy metal is mounted centrally in a bulb, and the electrons are directed upon it. The rays affect a photographic plate. They are absorbed to a greater extent by dense substances than by light ones, passing readily through cloth and flesh, but with less ease through bone and metal. This difference of penetrative power rendered them of immense value in surgery. When they were passed through hand or apn, or thinner parts of the body, a shadow of bones or denser substance, such as ring, needle, or bullet, was cast upon a fluorescent screen or photographic plate beyond. With modern apparatus the whole body can be photographed. Thus the whole alimentary canal, with the many twists and turns of the intestines, is rendered visible by giving the patient a meal of about 2oz of bismuth carbonate in lOoz of porridge, a meal so opaque to X-rays that a shadow of the tract as the food passes through it is thrown upon the plate. Diseases of the canal can thus be located for, treatment. For a long time the nature of the rays was in doubt, but it became evident that they were' electro magnetic, or light waves, with a wave-length much smaller even than the waves erf ultra-violet light. They differ also from ordinary light waves in not being continuous; they consist of short trains or pulses, produced by the impact of the electrons upon the metal anode. And just as waves of light enable us to investigate the structure of bodies through which they pass, so also the more minute wavelets in the X-rays provide us with an instrument of surpassing delicacy for the discovery of secrets which the grosser light-ray* fail to reveal. Radium, Again. 1 Shortly after the discovery of X-rays by Rontgen, Professor Becquerel, and Professor and Madame Curie, showed that salts of the very heavy elements uranium and radium affect the photographic plate, like X-rays, in the dark;! that the emission of these rays is independent of the temperature or of any influence which man can bring to bear, having started when he finds it, and going on gradually, uninterruptedly, in spite of him. It has been found that the radio-activity of these heavy, top-heavy, elements is due to their breaking down into lighter ones, giving off in the process a radiation containing both electrons and X-rays. This disintegration of radium into simpler elements, such as helium and lead, not only supplies a new weapon for trial in the war against cancer, but also shows man that electrons arc the bricks of which the universe is built. (To be continued next Saturday.)